To carry out many industrial processes and manufacturing operations it is often necessary to maintain the temperature and humidity of the working environment at specific preset temperature—humidity conditions or use clean steam in a process operation. In heating of homes and buildings low pressure steam is often used to heat the air in the central air heating systems.
In the forced air heating, ventilation and air conditioning of industrial, commercial, institutional and residential buildings the required humidity of the space air is most conveniently maintained by addition of the atmospheric steam into the recirculating space air. For such humidifying of air the generation of the atmospheric or low pressure steam on site is the most economic and often the only available alternative.
Technical and commercial literature indicate, that the current art compact isothermic humidifiers are producing the atmospheric steam by boiling and evaporating the make up water at a substantially atmospheric pressure in sealed water tanks. The required heat is produced either by electric power via two or more electrodes or resistance heating elements submerged in the boiling water, or by a pressure steam delivered from a central steam boiler to a submerged heat exchanger, or by combustion of natural gas in immersion tube burners in water tanks.
Disclosed in U.S. Pat. No. 5,816,496 is a gas fired steam generator—humidifier with an integrated combustion chamber—heat exchanger immersed in a rectangular water tank. It operates at substantially atmospheric pressure with a periodic flow of the make up water and a steam generation process including two operating periods. The first operating period involves steps including transfer of heat from the heat source into the boiling water, evaporation of the boiling water, concentration of dissolved solids in boiling water, separation of the produced steam from the boiling water and discharge of the produced steam from the water tank. The following second operating period, in addition to the above steps, includes filling up the water tank with make up water, heating the make up water to its boiling temperature and draining of a portion of the boiling water to maintain the concentration of dissolved solids in boiling water within a preselected concentration limits
Disclosed in my earlier application Ser. No. 08/657,179 entitled Compact Gas Fired Steam Generator, filed Jun. 3, 1996 and Ser. No. 09/419,577 entitled Compact Gas Fired Humidifier, filed Oct. 18, 1999 is a steam generator integrating a water tank with a tubular type boiler with a high rate internal reticulation of boiling water operating with a variable water level in the water tank, with a periodic flow of incoming make up water and with periodic blow down of boiling water with precipitated dissolved solids.
Disclosed in my earlier application Ser. No. 09/835,774, filed Apr. 16, 2001 entitled Compact Ultra High Efficiency Gas Fired Steam Generator is a compact gas fired steam generator integrating a water tank with a tubular type boiler with a high rate internal reticulation of boiling water operating with a constant water level in the water tank, with a substantially constant flow of incoming make up water, with a substantially constant blow down of boiling water with precipitated dissolved solids and with a condensing heat exchanger recovering latent heat from flue gases and with a heat exchanger recovering waste heat from the blow down boiling water.
The current art gas fired steam generators used in humidifiers available from Nortec Industries, Amstrong International, Inc. and DriSteem Humidifier Company produce steam from cold make up water in response to humidity demand of the air measured and controlled by a humidistat. They may produce the steam in cycles comprising an ON period followed by an OFF period maintained by an ON-OFF humidistat, or continuously at a variable rate on call from a modulating humidistat.
The used make up water may be a city water, softened water, deionized water (DI) or reverse osmosis treated water (RO). Most often the make up water is a city water containing dissolved solids and its flow into the water tank may be either substantially continuous or intermittent maintained by a suitable float valve or by a float switch operating a solenoid valve. As the feed water is converted to steam, impurities which enter with make up water are concentrated and the concentrated dissolved solids such as calcium and magnesium precipitate. Portion of precipitated solids accumulates as hard scale on the submerged heat transfer surfaces, the remainder settles out and accumulates on the bottom of the water tank which must be regularly cleaned to maintain its operation.
To minimise build up of hard scale on the heat transfer surfaces, the current art humidifying systems drain a portion of the boiling water to maintain the concentration of dissolved solids in boiling water within a preselected concentration limits or use ionic beds (Armstrong) that must be regularly replaced.
The overall thermal efficiency of the current art gas fired steam generator used in humidifiers is low and in the range of 75%-80%.
To provide the required heat transfer area, the current art gas fired steam generators require relatively large water tanks with large volume of boiling water to accommodate the required heat transfer surfaces causing a delayed response in steam production.
To protect the water tank against an accidental increase in operating pressure the steam generators are provided either with a water seal or a pressure relief valve. The water seal also provides for regular periodic overflow of the blow down boiling water while preventing steam from escaping through the overflow outlet. With such overflow water seal arrangement, the steam pressure in the tank is limited by the height of the water seal column in the overflow tube.
The method of production of steam in current art gas fired steam generators involves steps including: feeding cold make up water into the water tank, burning fuel with air to produce combustion products, transferring heat from combustion products by heat exchanger immersed in the boiling water to preheat and boil the make up water to produce steam, concentrating and precipitating dissolved solids in boiling water or removing dissolved solids by ionic beds (Amstrong), separating the produced steam from boiling water, discharging the produced steam from water tank, regularly blowing down boiling water to maintain an acceptable concentration of dissolved and suspended solids in boiling water, regular cleaning of heat transfer surfaces and replacing the ionic beds and regular removal of accumulated solids from the water tank.
There are three groups of controls that are integrated to maintain the operation of the current art gas fired humidifying systems fully automatic. These include: a) humidifying controls regulating the humidity of air, b) combustion controls regulating the combustion of fuel in response to humidity demand of the air and c) the water controls regulating the flow of make up water, water level in water tank, blow down of the boiling water, and draining of the water tank. Monitoring and display of selected performance parameters is also provided with some current art systems.
A major concern with the current art gas fired steam generators are the large water tanks with a delayed response in steam production, high initial cost, high operation maintenance cost, excessive consumption of water due to required regular blow down of boiling water and low overall thermal efficiency.
The delayed response in steam production is due to large water tanks required by the heat transfer surfaces, the high initial cost is due to use of relatively large stainless steel water tanks and large stainless steel heat exchangers, the high operation maintenance cost is due to required regular cleaning of heat transfer surfaces and replacement of ionic beds or manual removal of accumulated settled solids from water tanks and the low overall thermal efficiency is due to regular blow down of boiling water and low combustion efficiency due to loss of latent heat with discharged combustion products.
Therefore, to overcome the shortcomings of the current art fuel fired steam generators the object of the present invention is to provide a compact steam generator and a method for producing steam from cold make up water containing dissolved solids with reduced volume of boiling water in the water tank, with a heat exchanger submerged in the water having extended heat transfer surfaces with reduced heat transfer resistance on the side of combustion products and with reduced build up of hard scale, with recovery of latent heat from cooled combustion products without the need for an additional heat exchanger and without the need for the regular blow down of boiling water or use of ionic beds and without the need for regular manual removal of settled solids.